Electroluminescent lamp having a high resistivity electrode



Oct. 31', 1967 BURNS 3,350,596

ELECTROLUMINESCENT LAMP HAVING A HIGH RESISTIVITY ELECTRODE Filed Aug. 1'7, 1953 HIGH RESIST/WT) ELECTRODE HIGH RESIST! W7 Y ELE C TRUDE INVENTUR United States Patent 3,350,596 ELECTRGLUMINESCENT LAMP HAVING A HIGH RESISTIVITY ELECTRODE Laurence Burns, Swampscott, Mass, assignor, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware Filed Aug. 17, 1953, Ser. No. 374,771 Claims. (Cl. 313-108) This invention relates to electroluminescent lamps, that is to lamps in which luminescence is produced by the action of an electric field on a phosphor.

An object of the invention is to increase the brightness and efliciency of such lamps.

When the phosphor is embedded in a solid dielectric material, such as a plastic or a glass, and low-resistance plane electrodes placed on opposite sides of the resultant material, there will be a general luminescence in the entire area between the electrodes, but if the voltage is raised sufiiciently there will also be a series of brighter points, scintillating like sparks from point to point in the material.

Such sparking will occur only once at a particular point, a dim spot being left after it, if one of the electrodes is very thin, such as an evaporated metallic film would be. The spar vaporizes the metal film away at the point of sparking. The vaporization can be prevented by using a heavier metallic film or electrode, but in that case the dielectric layer will crack if glass is used, or will be decomposed if a plastic is used, because of the high current that can then flow in the region of electrical breakdown.

In order to get the advantage of the tremendously higher brightness at the sparking point without the loss of either the electrode material or the dielectric material, it is necessary to limit the current through the sparking points. The use of an impedance in series with the lamp will limit the total current to the lamp, and could be used to limit the current if the dielectric-phosphor layer and the electrodes were absolutely uniform throughout the active part of the lamp, but such uniformity is diflicult to attain in practice, because the layer is generally only a few thousandths of an inch thick and even a minute deviation from that thickness would change the field greatly at the deviating spot. The current through the whole lamp might not be changed greatly by breakdown at a particular point, but the current at the points of breakdown would increase sufficiently to destroy either the electrode or the dielectric-phosphor layer in that vicinity.

If, however, the electrode materials are of suflicient resistivity, then the rise in current at the breakdown points will be limited, because the additional current will have to pass through a region of high resistivity to reach the breakdown point. This is especially true if the electrode is not only of high resistivity but is also thin, so that current flowing to the breakdown point from surrounding points'will have to pass through the material transversely to the thickness of the dielectric phosphor layer. The resistance will then be almost a surface resistivity, and can be expressed in the usual manner for surface resis tivity, that is, in ohms per square, which means the resistance taken between opposite ends of a square film of the desired thickness. By a high resistivity coating, -1 mean one whose surface resistivity is at least one order of magnitude higher than the maximum value of 10' ohms per square ordinarily used.

A film of the various transparent and electrically conductive coatings used on glasses, for example of tin chloride or the like, which appear to be semi-conductors, would provide such a high resistance. If used on only one 'side of the dielectric-phosphor layer, however, the coating will not be fully effective, because the current from several points on the high-resistivity side will still be able to converge on a single point on the other side, unless the other side be also of high resistivity, or be surrounded by a sufficien-t body of material to distribute the heat. Except under the latter condition, a high resistivity coating should be used on both sides of the dielectric-phosphor layer for best results. The high resistivity coating need not be transparent on both sides, but the coating on one side at least can be evaporated, painted, sprayed or otherwise applied to the film of high resistivity metal, semi-conductor or the like, which may be, for example of carbon, although the use of carbon may reduce the brightness somewhat by its lack of reflective properties. A protecting layer of enamel or the like may be necessary to prevent evaporation at sparking, if the layer is very thin.

In some cases, to remove the possibility of breakdown in the dielectric material and to confine the breakdown to the phosphor, the dielectric material can be eliminated and a layer of powdered phosphor, preferably somewhat compressed, used between the electrodes, as shown for example in my copending application, Ser. No. 305,400, filed Aug. 20, 1952. In that case one electrode could be a transparent conductive coating on glass or the like, and the other a metal coating applied over the compressed layer by evaporation, spraying or the like, care being taken to prevent the metals depositing in a direct path through the phosphor, to prevent short-circuiting.

The phosphor coating can be applied as in my copending application above-mentioned, or can be applied by eing mixed with a suspending solution and the suspending material afterward removed by drying or baking, in the manner customary in the fluorescent lamp art.

When the phosphor is used without the embedding dielectric material, the voltage necessary will be smaller if the work function of the contact metal used is high. The operation of the device may be improved by depositing spots of metal or other conducting material, on the phosphor particles, for example as shown in my copending application above-mentioned. In that case also, the operation will be improved by the use of metal or other materials of high Work function.

If substantially a single layer of crystals is used between the electrodes, and if the material of one electrode has a higher work function than that of the other, or if one electrode is made of a hard, firmly plane material to give a small area of contact with the crystals, the other electrode being soft, or deposited by evaporation or like to give a larger area of contact with the phosphor crystals, a rectifying device will result, and will be particular useful in applications where one or more of the electrodes is in the form of a series of insulated lines or strips so that a desired pattern can be produced over the surface of the phosphor.

If one of the contacts to the crystal is a point contact, an external resistance can be used with the device, but in that case the area of luminescence will be small. Several such contacts can be used over the surface of the crystal, or over the surface of a layer of dielectric material in which phosphor crystals have been embedded, if desired.

Where the resistivity required for the above results is high enough to reduce the voltage so much from point to point as to make the luminescence non-uniform over the electrode area, the high-resistivity layer can be backed by a low-resistivity layer, the resistance of the high-resistivity layer then being the resistance to current passing transversely through the layer, instead of to current passing along the layer. In that case the layer will generally need to be thicker than before to obtain suflicient resistance in the current path.

The low-resistance layer, which may be of metal, can be applied over the high-resistance layer by evaporation,

spraying, painting or the like, but it will often be convenient to apply it in the form of a metal plate having a roughened surface to provide a number of separated contact points with the high-resistance layer, to increase the path of the current through the high-resistance coating. In the case where the high-resistance coating is applied to a layer of phosphor particles which are not embedded in a dielectric medium, the high-resistance coating will have a rough surface of its own and the mere application of a lower-resistance metal plate to it will of itself restrict the contact to separated points.

The high-resistivity layer will serve, in any case, to even out the field in the phosphor layer and to allow a voltage of the order of the breakdown voltage to be applied all over said layer, thus spreading out the high luminescence of the sparking points to a larger area of the phosphor layer. The high brightness of sparking appears to occur when the voltage acquired by an electron travelling in the phosphor particle is sulficient to excite the main lattice atoms to energies of the order of 2 volts or so, that energy being then transferred to the activator atoms or ions which eventually release it as light. Energy can also travel along from one lattice atom to another until it reaches the surface of the particles, where the lattice atom or ion can be de-energized with emission of light or other radiation.

The electrons, which may be free electrons coming from donator atoms or the like in the phosphor particle, or from field emission in the phosphor or at the contact with the phosphor, will oridinarily be accelerated to transferrable energies when dropping down a potential energy hill of energy fall sufficient for visible excitation, that is about 1.5 to about 3 electron-volts. The energy fall must occur in a distance over which the electron can be accelerated to that much energy, that is in a distance short enough compared with the lattice spacing to enable the electron to acquire that much energy in a reasonably short path. If the required energy drop occurs across too long a distance, the electron will lose energy as heat to the lattice by collision faster than it can gain energy from the field.

If the phosphor particles used contain acceptor atoms, the latter will free so-called postive holes and the latter can then be accelerated as the electrons were. In that case, however, the activator atoms or lattice atoms will be excited to visible energies on collision with holes which have floated up a potential hill of the required number of electron volts, say about 1.5 to about 3 electron-volts There will ordinarily be a high field and sulficient voltage drop in a sufficiently short distance in the phosphor particles when the latter is in contact with a metal or even with another semi-conductor, the phosphor itself being one semi-conductor. A depletion layer on all or part of the surface or at other spots in the phosphor will also produce such a high field and high voltage drop, a depletion layer here meaning a layer in which the density of fixed donator or acceptor atoms or ions, or the like, is smaller than in the main body of the particle.

Other objects, advantages and features of the invention will be apparent from the following specification taken in connection with the accompanying drawing, in which:

FIG. 1 is a schematic view of one embodiment of the invention.

FIG. 2 is a schematic view of another embodiment.

FIG. 3 is a sectional view of an electrode of one embodiment.

FIG. 4 shows a crystal with a depletion layer.

In FIG. 1, the glass plate 1 carries the transparent conductive coating 2 over a portion of its surface. A layer 3 of electroluminescent phosphor particles embedded in a solid dielectric medium extends over part of the transparent conductive coating 2, and a backing layer 4 of high-resistivity material extends over a part of the phosphor-dielectric layer 3. To prevent flashover through the air between layer 2 and 4, the layer 4 terminates at a distance from the edges 5 of the layer 3. The layer 3 in turn extends over the edge 6 of the conductive layer 3, to insulate the layer 4 from layer 2.

In FIG. 2, the glass plate 1, transparent conductive coating 2, phosphor-dielectric layer 3 and high-resistance layer 4 are the same as in FIG. 1, except that the top surface 7 of the phosphor-dielectric layer 3 is rough or wavy, the high-resistance layer 4 conforming to it and therefor being also rough or wavy, thereby contacting the low-resistance layer 8 only at a series of separated points. This increases the resistance between layer 8 and various points on the surface 7, thereby stabilizing the lamp at high voltage and preventing localized sparking. If the dielectric -is omitted in the layer 3, so that the layer 4 consist only of powdered phosphor particles, and the resistive layer 4 applied directly onto the phosphor layer 4, it will of necessity be of a rough irregular pattern, at least if viewed under sufiicient magnification.

The layer 4 can be applied by vacuum deposition of metal, for example, of aluminum or tungsten over the layer 3 of phosphor particles, care being taken to insure that the metal does not seep far enough through the layer 4 to cause short-circuiting.

A clamping rim 9, 10 of metal may be used around the edges of the device to hold the parts together, care being taken to avoid short-circuiting effects.

In some cases, it may be desirable to enclose the device in an hermetically sealed envelope and evacuate the same to increase the voltage which can be appplied to the device without breakdown in the surrounding air.

A voltage sulficient to produce an electric field of 10' to 10 volts per centimeter at certain points in the crystal will give a very bright phosphor, although luminescence will be obtained at lower fields.

The layer 3, if of phosphor particles alone can be applied by mixing the phosphor particles with a suspending medium of the type used in coating phosphors onto fiuorescent lamp tubes, for example by mixing 3% ethyl cellulose with xylol and 7% alcohol as shown in U.S. Patent 2,524,733, granted to Elmer F. Payne et al., on Oct. 3, 1950. After the coating has dried, it is baked at a temperature sufiicient to decompose the cellulose and burn it off, a temperature of about 450 C. in air being generally satisfactory.

The high resistance coating 4, in either FIG. 1 or FIG. 2, can be a layer of carbon, conveniently painted onto the coating by methods well known in the art.

The transparent conductive coating 2 can be of stannic chloride or the like, for example as shown in U.S. Patent 2,624,857 to Eric L. Mager.

In some cases, especially with direct conductive contact to the phosphor particles, the phosphor may show a negative resistance on high voltage, as in a gaseous discharge, and under such conditions the ballasting effect of a series resistance is especially desirable. If the phosphor particle is made large enough, a direct contact may be made to a single particle, with a ballasting resistance to limit the current when the voltage is made sutficient to give the particle a negative resistance.

FIG. 3 shows an alternative construction for the highres'istance layer 4 of FIGS. 1 or 2. The conducting filaments or wires 11 of carbon, tungsten, nichrome or other high resistance material, extend through the insulating plate 26 which may be of glass or other insulating material, for example, a plastic such as urea formaldehyde. The wires extend slightly out of each side of the insulating plate, and can be connected together at one end, for example by contacting the metal plate 4, and at the other end can contact the phosphor layer 4 (or phosphor-dielecric layer) at a multitude of points, each separately ballasted by a resistance wire 11. The wires 1 are very fine, for example, No. 40 B. & S. gauge or even less.

The layer 4 can also be made of a glass which is conductive throughout its body, as distinguished from a socalled conducting glass which merely has a conductive surface layer 2, the remainder of the glass piece 1 being insulative. Glasses which are conductive throughout their body can be made by fusing a mixture of a glass and semi-conducting spinels as is known. Such glass can also be made by converting a low melting glass to frit, grinding it very fine, mixing it with finely powdered graphite, spraying onto porcelain, and fusing it, the latter being generally done at a temperature of 800 C. to 900 C. Resistors of this can be made in low or high resistance, depending on the concentration and size of the graphite particles. Resins can be used with the graphite instead of glass.

Alkali-containing glasses also conduct current, by ion transport or electrolysis.

The phosphor in layer 3 can be of any electroluminescent type, for example, a tin-containing zinc manganese silicate, a manganese-activated zinc fluoride, a Zinc or cadmium sulfide or selenide, activated with copper, or with copper and manganese, and containing a little lead, for example, as shown in the copending application Ser. No. 230,711 filed June 8, 1951, by Keith H. Butler.

Where the phosphor layer 4 also contains a dielectric embedding medium, the medium can be of glass, of plastic such as a melamine-alkyl resin or of nitrocellulose, for example, as shown in US. Patent 2,566,349 issued Sept. 4, 1951, to Eric L. Mager, or as in application Ser. No. 298,387 filed Nov. 14, 1951, by Keith H. Butler.

The phosphor used is generally washed with a material such as acetic acid or ammonium acetate, especially if the phosphor is zinc sulfide containing some oxide, the washing removing the low conductivity zinc oxide particles. The washing also produces or enhances a surface depletion layer on the phosphor particles, making them effective for excitation in an embedding medium. When direct contact is made to the crystals, the depletion layer is not always necessary, and in fact if direct contact is .made at low voltage to a copper-activated zinc sulfide .having a depletion layer, the phosphor will glow less brightly then if the depletion layer were not present, or

than if the contact were made to a portion of the crystal on which the depletion layer is not present. It is found that a phosphor which electroluminesces green when contact is made through the depletion layer, electroluminesces bluish-white when contact is made directly to the main semi-conducting body of the crystal, without going through the depletion layer. This appears to be due in part to the presence of less activating-copper in the main body of the crystal, although copper can also be present in the main semi-conducting body of the crystal as donator or acceptor atoms or ions. If the field is high enough, there will also be the effect of the free electrons being accelerated sufiiciently to excite the main lattice atoms (for example, zinc in zinc sulfide) which then transfer this energy to the blue-emitting levels because they are nearer in energy to them than to the green-emitting levels. However, since it takes at least 3.5 electron-volts to excite the main lattice, and less than that to excite an activator, the proportion of main lattice atoms excited to energies suificient for visible emission will be small except at very high voltages. V

On raising the voltage, however, better results are secured with the presence of a depletion layer, even in the direct contact cases.

The overall resistance of the particle drops from being of the order of to 10 ohm-centimeter with the depletion layer of the order of 10 to 10 ohm-ems. without the depletion layer, that is without the depletion layer at the luminescing portion.

When an embedding dielectric which attacks the depletion layer is used, the lumen output of the lamp will generally rise for the first few hundred hours, to about 130% to 150% of its initial brightness, and will then fall off slowly through the next few thousand hours. This is the general effect, with the usual washing treatment given to the phosphor, and appears to be due to the gradual reducthe contact of the proper polarity for added at the same time by including 'vaporized copper salt in the sulfur atmosphere, or by tion in thickness of the depletion layer. However, if the washing treatment is reduced in intensity or in time, so that the depletion layer formed is not eaten away by the treatment, then the lamp will rise in brightness for a longer period before dropping off, although its initial brightness will be somewhat less. On the other hand, a more intensive or extensive washing treatment may bring the depletion layer to its optimum thickness at the beginning of lamp life.

Washing the phosphor in an alkaline solution, for example ammonia, after the initial acidic or ammonium acetate treatment, will generally help to stabilize the phosphor during life, and reduce the rise or drop in light output.

When the embedding dielectric medium is omitted, the layer 4 then being composed merely of phosphor particles, the device can be operated on direct current, With direct contact to the crystals. A high voltage will generally be required to get good general luminescence throughout the layer 4, because of the presence of a depletion layer on the surface of each crystal.

There will generally be some spots on each crystal which are free of the depletion layer, and if contact is made to such spots, the crystal will luminesce brightly at lower voltage. The efiiciency, however, will generally be greater if the contact is made to the depletion layer, so that the depletion layer becomes luminescent, because there is ordinarily a somewhat higher concentration of copper activator atoms in the depletion layer.

In FIG. 4, the semi-conducting crystal core 11 has the depletion layer 12 over its surface. The contact 13 is shown as being made to the depletion layer 12, the contact 14 as being made to the main semi-conducting core 11 through a break in the depletion layer 13. At low voltages, it will be best to make both contacts directly to the semi-conducting core 11, possibly dispensing with the depletion layer altogether, although one of the contacts can still be made through the layer 12, but will preferably be of the polarity opposite to that which produces lumines-cence, if D.C. be used, the luminescence then occurring at the direct contact 14 to the semi-conductor core 11. With higher voltages, both contacts, or on DC. at least luminescence, can be made to the depletion layer 12.

In FIG. 4, the contacts 13 and 14 may be metals or may be suitable semi-conductors, materials of high work function generally being desirable. They can be either point contacts or contacts of large area, and if the crystal is used in an embedding dielectric can be spots of metal or of good semi-conductor material present on the surface of the crystal. However, if the crystal is embedded in a dielectric medium,'the presence of a depletion layer will also produce luminescence at projecting points like point 24, where a high field will be present in the depletion layer. It will also produce luminescence at points of contact with another crystal 25 which can have a depletion layer of its own.

Some electroluminescence can be obtained from particles having an internal core of metal, with a thin phosphor layer over the outside of the particle. Such particles 7 can be made, for example, by heating particles of metallic zinc in an atmosphere of sulfur at a temperature such that the zinc reacts with the sulfur, and continuing the process only until a surface layer 4 of sulfide is produced on the particle. The copper or other activator can be a small amount of afterward heating the zinc particles with copper chloride, or in atmosphere of the latter. Some chlorine appears to be helpful to the luminescence of sulfides, and so a little chlorine might also be added to the sulphur atmosphere, or to the particle afterward. The use of such metalliccore particles will generally require a higher voltage than the particles with a semi-conductor core, because field or thermal emission from the metal supplies the elec- 7 trons. very small amount of lead may be added to the phosphor layer on the metal core.

It may be desirable in some cases to evaporate activated zinc sulfide or the like and to condense it directly over a surface of considerable area, for example, over a flat transparent conductive surface on glass, with a conductive surface then evaporated over the sulfide layer. In such a case, a high resistivity conductive surface may be very helpful in evening out the field over the surface. After depositing the activated zinc sulfide on the surface, it may be heat-treated to produce a depletion layer on the surface, and afterward treated with acetic acid or other chemical agent to remove any low-conductivity material which may have formed on the surface, as is done with powdered phosphor particles.

A conducting surface may then be placed over the depletion layer on the zinc sulfide by evaporation or the like.

In the embodiment of the foregoing paragraph, the phosphor layer 3 in FIG. 1 may be continuous and uniform rather than being formed of a multitude of discrete phosphor particles, or of discrete particles embedded in a dielectric material. However, if the discrete phosphor particles are used, the voltage across them can be increased by placing particles of some substance with an extremely high dielectric constant, such as barium titanate, between the particles either directly, if no other embedding dielectric is used, or in the embedding dielectric, if one is used. The barium titanate particles can be of much smaller particle size than that of the phosphor particles. For example, if the phosphor particles are about 20 microns in extent, the titanate or similar particles can be as small as 1 micron or even less.

In the case where a high field is desired at the phosphor contact with a metal, the metal used should have a low work function if the phosphor portion in contact with it, has p-type conductivity, that is, if its conductivity is due to holes rather than to electrons.

In some cases where the phosphor is embedded in a medium, it will be desirable to embed it in a conductive medium rather than in a purely dielectric medium, the medium having a conductivity about equal to the average overall conductivity of the phosphor, including its contact resistance.

While the resistance or resistivity of the coatings and the like described herein should be high enough to prevent or reduce spot breakdown, it should not be high enough to dissipate enough power to seriously impair the efiiciency of the phosphor. The surface resistivity where a thin surface coating is used, should not ordinarily be greater than 10 ohm-ems, and will generally be between about 10 and 10 ohm-cms.

What I claim is:

1. An electroluminescent lamp comprising two electrodes with a layer therebetween of an activated phosphor particle containing an acceptor impurity and being embedded in an insulating dielectric medium.

2. An electroluminescent lamp comprising one electrode, a layer of electroluminescent phosphor thereover, a layer of glass conductive throughout its body over said phosphor layer, and an electrode over said conductive glass layer, the resistivity of said electrode being lower than that of the conductive glass.

3. The lamp of claim 2, in which the resistivity of the conductive glass layer is between and 10 ohm-ems.

4. An electroluminescent lamp comprising an electroluminescent phosphor crystal, a source of voltage high enough to make the resistance of said crystal negative when impressed across it, an impedance to limit the current through said crystal despite the negative resistance of the latter, and connections to impress said source of voltage and said impedance across said crystal in series.

5. An electroluminescent lamp comprising an electro luminescent crystal, a source of voltage sufiicient to apply an electric field of between 10" and 10 volts per centimeter to at least part of said crystal, and means connecting said source of voltage to said crystal.

6. An electroluminescent device comprising a layer of electroluminescent material mixed with barium titanate disposed between a pair of electrically independent electrode layers.

7. An electroluminescent device comprising a layer of a mixture of electroluminescent material and barium titanate dispersed and suspended in a dielectric medium, said layer being disposed between a pair of electrically independent electrode layers.

8. An electroluminescent device comprising a base member, a transparent layer of electrically conductive material disposed on said base member, a layer of a mixture of electroluminescent material and barium titanate disposed on said transparent layer, and a layer of electrically conductive material disposed on said layer of said mixture, said layers of electrically conductive material being electrically independent.

9. An electroluminescent device comprising a base member, a transparent layer of electrically conductive material disposed on said base member, a layer of a mixture of electroluminescent material and barium titanate dispersed in a dielectric medium and disposed on said transparent layer, and a layer of electrically conductive material disposed on said layer of said mixture, said layers of electrically conductive material being electrically independent.

10. Capacitor-phosphor crystal comprising a phosphor crystal, a dielectric overall covering the phosphor crystal, a plurality of separate conductive layers over the dielectric covering, said separate conductive layers arranged in a capacitor structure.

References Cited UNITED STATES PATENTS 2,566,349 9/1951 Mager 313108.1 2,624,857 1/1953 Mager 313108.-1 2,716,298 8/1955 Spielmann et al. 313l08.1 X 2,721,950 10/1955 Piper et al. 313108.1 2,765,419 10/1956 Roberts 313108.1 2,780,731 2/1957 Miller 313-1081 2,841,730 7/1958 Piper 313--108.1 FOREIGN PATENTS 717,169 10/ 1954 Great Britain.

OTHER REFERENCES Physical Review; vol. 87, No. 6, Sept. 15, 1952, pp. 151, 152.

S. D. SCHLOSSER, Primary Examiner.

GEORGE N. WESTBY, ARTHUR GAUSS,

RALPH G. NILSON, Examiners.

C. R. CAMPBELL, E. G. GERMAIN, L. D. BULLION,

Assistant Examiners. 

1. AN ELECTROLUMINESCENT LAMP COMPRISING TWO ELECTRODES WITH A LAYER THEREBETWEEN OF AN ACTIVATED PHOSPHOR PARTICLE CONTAINING AN ACCEPTOR IMPURITY AND BEING EMBEDDED IN AN INSULATING DIELECTRIC MEDIUM. 